The authorial voice is best defined as
the voice used by authors when seeming to speak for themselves
the voice that expresses the historical author's opinions
the fictional narrator that may have a different personality, background, or viewpoint from the historical author
the attitude the author or narrator has toward the subject matter

In this question, the authorial voice is best described as the voice used by the voice used by authors when seeming to speak for themselves. The historical author is the one writing the text and his or her opinions may or may not be in the text itself; the "author," meanwhile, is the one who the reader perceives to be behind the narration. The fictional narrator is separate from the author and often has a different personality or point of view altogether.